Biaxially molecular-oriented polyethylene terephtalate is small in permeability to gases such as oxygen, carbon dioxide or the like and is excellent in heat resisting property, rigidity and transparency, and thus the polyethylene terephtalate is, at present, widely used as a material for plastic bottles for carbonated drinks.
The pressure resisting property and shock resisting property of biaxially oriented plastic bottles (hereinafter referred to as bottles) are obtained when uniform molecular orientation is provided over the whole bottle, and the satisfactory heat resisting strength and shock resisting strength cannot be obtained for bottles in which molecular orientation is not uniform. When a bottle having a conventional bottom structure produced by stretching blow molding techniques presently employed is filled with carbonated drinks and sealed, and when the bottle is left at a temperature of 38.degree. C., the bottom surface of the bottle is deformed by internal pressure and bulges. By increasing the thickness of the bottom wall to prevent such a deformation, the shock strength is reduced whereby if the bottle falls from a high shelf, the bottom breaks. This is because of the fact that in view of the structure of bottles, the biaxial orientation of the bottom tends to be insufficient so that effective molecular orientation may not be obtained.
Therefore, in bottles for carbonated drinks, particularly in bottles for cola, the bottom is formed into a spherical configuration which can be easily biaxially oriented and a base cup formed of other plastic is mounted thereon to provide self-standing properties. Said mounting of the base cup requires adhesives and is accomplished by use of a device entirely different from the bottle molding machine. Therefore, this process increases manufacturing cost as compared with the case of molding bottles having the self-standing properties, and when bottles formed of polyethylene terephtalate are reused, base cups have to be removed every time of operation. In bottles of small capacity, 500 ml, the ratio in which base cup is occupied is greater than that of large bottles in excess of 1000 ml, thus increasing the cost.
In view of the foregoing, self-standing bottles for carbonated drinks which require no base cup have been developed, a part of which is now under operation. Free standing bottles commercially available at present includes those which have petaloid leg portions in which semispherical bottom wall portions are partially projected outwardly (U.S. Pat. No. 3,598,270 Specification) and those which comprise an outer peripheral wall portion in which the bottom wall portion is inwardly inclined and a dome-like bottom wall central portion inwardly curved from the lower end of said outer peripheral wall portion, said outer peripheral wall portion having a plurality of leg portions outwardly projected (British Patent Laid-open No. 2,040,256A Specification).
Any of these aforementioned free standing bottles have leg portions projected, by which leg portions the free standing properties may be secured and the bottom portion is prevented from being deformed due to internal pressure by the rib formed between said leg portions or the rim in the peripheral edge of the dome. In such bottles, it is difficult to mold leg portions, and molding technology of high degree and molding conditions are necessary in order that all of the required number of legs formed therein may be formed in the same form. Further, the free standing properties obtained by the leg portions involve no particular problem in the stationary state of bottles but when bottles are moving for the purpose of being filled, bottles often fall because of slight offsets between the bottles.
Bottles of new structures which overcome these disadvantages noted above with respect to the aforesaid free standing bottles have been proposed (Japanese Patent Application Laid-open No. 163,137/1980). This bottle is provided with the bottom structure similar to that of a champagne bottom, and the free standing properties of the bottle is secured by annular portions in contact with the ground formed over the lower ends of an outer peripheral wall portion in which the bottom wall portion is inwardly inclined and an inner peripheral wall portion in the form of a truncated cone of said outer peripheral wall portion, and the pressure resisting properties thereof is secured by the inner peripheral wall portion and a central portion of the bottom wall having a convex curved surface continuously formed thereabove.
In the bottom structure (hereinafter referred to as the champagne bottom) of said newly proposed bottle, when the bottle, if it is filled with contents, falls, the bottle is broken circularly along the annular portions in contact with the ground. This results from the small radius of curvature of the annular portions in contact with the ground and insufficient biaxial orientation made when molding. Thus, the annular portion in contact with the ground is further projected into a semi-circular shape in section to provide the large radius of curvature, but it involves a difficulty in projecting the annular portion in contact with the ground which corresponds to the juncture between the outer peripheral wall portion and the inner peripheral wall portion into a semi-circular shape in section over the entire circumference similar to that of molding leg portions as already mentioned, and when the radius of curvature increases, the falling shock strength increases but the pressure resisting strength decreases whereby the central portion of the bottom is deformed due to the internal pressure.
The present inventor has molded champagne bottoms of various structures and studied as to the pressure resisting strength and falling shock strength thereof, and as the result, the present inventor has found that to meet the conditions required for bottles for carbonated drinks, that is, conditions such as the pressure resisting strength, falling shock strength, weight reducing, low cost of molding and the like, it is preferably that the angle of inclination formed between the outer peripheral wall portion and the inner peripheral wall portion be formed as acute as possible, the sectional shape of the annular portion in contact with ground is not semi-circular but the curvature between the outer peripheral wall portion side and the inner peripheral wall portion side is made to have a difference, and the wall thickness of the bottom wall portion is made gradually thicker from the outer peripheral wall portion towards the bottom wall central portion, the bottom wall portion being molded at a low temperature of parison and at a low blow pressure.
It has been also found that it is difficult for the prior art stretching blow-molding method, which is a method for axially stretching a parison while all of blow molds are secured and fully distending it in the cavity, to mold a champagne bottom satisfied with the aforesaid conditions. This difficulty results from the fact that a parison of low temperature cannot be stretched, at a low blow pressure, into a narrow space for molding an annular portion in contact with ground formed in the periphery of a bottom mold.
A molding method capable of being satisfied with the above-described conditions comprises imparting a positional change to a bottom mold which forms a part of a blow cavity, stretching and blowing a parison fully to a point where the bottom mold is positioned for molding a bottom portion to precede molding of an outer peripheral wall portion, moving the bottom mold to raise bulged out of the cavity, pushing back the bottom between the lower end of the cavity and the bottom mold to the outer peripheral wall portion, and forming the inner wall portion and bottom wall central portion as if they are turned back into a bottle.
The above-mentioned molding method is disclosed in U.S. Pat. No. 4,035,455 Specification, but it is the most important in this invention that the bottom mold be moved while the bottom of the parison still retains a deformation temperature.